This invention relates to the regeneration of a catalyst comprising iron, phosphorus, oxygen and possibly one or more other elements such as the alkali or alkaline earth metals, which is preferably used in the oxydehydrogenation of isobutyric acid to form methacrylic acid.
These catalysts commonly referred to as iron/phosphate catalysts are well known as mild oxydizing catalysts. The production and use of catalysts of this type is disclosed in Cavaterra, U.S. Pat. No. 3,948,959. Primarily, the catalyst contains iron, phosphorus and oxygen. The iron in the catalyst is a mixture of Fe.sup.++ and Fe.sup.+++. During use, typically to oxydehydrogenate a lower molecular weight organic compound such as isobutyric acid, the iron in the plus 3 state is reduced to iron in a plus 2 state. This causes a deactivation of the catalyst. Over a long period of use, the catalyst tends to be less effective both in terms of conversion rate and selectivity.
A typical solution to this problem has been to cut off the feed of organic material into and through the catalyst bed and to maintain the flow of steam and air or steam and oxygen at or above operating conditions over a period of time. This tends to return much of the Fe.sup.++ to Fe.sup.+++. However, not all of the iron does change to Fe.sup.+++. This is critical in that the catalyst seems to require iron in both states in order to function properly. However, of course, as this oxidizing reaction is once again resumed, the Fe.sup.+++ is reduced to Fe.sup.++ and the catalyst again slowly deactivates.
It has been found quite unexpectedly, and contrary to the theory behind the iron phosphorus catalyst, that the catalyst can be regenerated to a higher state of efficiency by first discontinuing the flow of organic feed material and continuing the steam-air or steam-oxygen flow, thereby creating an oxidizing atmosphere, and subsequently changing the atmosphere in the reactor bed to a mild reducing atmosphere, preferably by introducing only organic feed and steam in the absence of air or oxygen for a period of time. This two step procedure has been found to significantly improve the efficiency of the catalyst, especially in view of the prior art.